The present invention is related generally to medical devices. More specifically, the present invention is related to epidural needles or cannulas. The present invention includes an epidural needle having a distal flare for providing an increased resistance to penetration, and a greater sense of decrease in resistance to penetration, during and after the needle tip has penetrated the ligamentum flavum, respectively.
Injection of fluids into the epidural space involves careful placement of the tip of the epidural needle in the epidural space. Commonly injected fluids include anesthetic agents. Such agents are commonly injected into the epidural space during childbirth to block pain. A plastic catheter may be inserted through the needle into the epidural space and the needle withdrawn for continuous infusion of medications through the catheter. The spinal epidural space is a potential space that extends from the base of the skull to the tail bone or coccyx. Approached from the back, the epidural space is deeper than the ligamentum flavum that connects the vertebral arches, but outside of the dural membrane that covers the spinal cord. This potential space, which varies in width from a few millimeters to a centimeter, normally contains loose areolar tissue, fat, and veins.
It is important to properly locate the epidural space prior to injecting fluid. If the needle penetrates too far, the dural membrane will be punctured, which can result in severe and long lasting headaches. The most commonly used technique to localize, or locate, the epidural space is by testing for the loss of resistance to injection of air or saline. The epidural needle, with a stylet disposed within, is inserted in the midline between the spinous process of two vertebrae and advanced a few centimeters towards the epidural space. Once the needle is embedded in the ligamentous structures, the stylet is removed, and a syringe containing a few milliliters of air or saline is attached to the hub of the needle. The resistance to injection using the syringe is checked either intermittently or continuously as the needle is slowly advanced through the ligamentous structures. Since the tip of the needle is buried in dense ligamentous structures, it will be hard to push the plunger of the syringe. As the needle passes ventral to the ligamrentum flavum, it enters the epidural space and there is a sudden loss of resistance to injected air or saline. A few more milliliters of fluid may be injected to confirm the relatively low pressure in the epidural space.
The epidural space may also be reached through a paramedian puncture. Here the epidural needle is inserted a few centimeters lateral to the midline and advanced towards the epidural space. The needle tip travels through the paraspinous muscles before it punctures the ligamentum flavum. Just as in the midline approach, there will be a loss of resistance to injection of air or saline once the needle tip enters the epidural space. In both the midline approach and the paramedian approach, the loss of resistance to injection of air or fluid is the key to localizing the epidural space.
Another technique described in the literature to localize the epidural space is the xe2x80x9changing dropxe2x80x9d method. (Bromage P R: Epidural Anesthesia. Philadelphia, W B Saunders 1978). A drop of solution is placed within the hub of the epidural needle. When the needle is advanced into the epidural space, the solution is sucked into the shaft of the needle. The theory attributed to this maneuver has been the presence of, or the creation of a subatmospheric pressure in the epidural space by the advancing needle tip. The presence of negative intrathoracic pressure, and the moderate expansion of epidural space by the needle pushing the dura away from the ligamentum flavum, have both been attributed to the low pressure responsible for sucking in the hanging drop.
Either method of localizing the epidural space, the xe2x80x9closs of resistance to injectionxe2x80x9d, or the xe2x80x9changing dropxe2x80x9d technique, relies on identifying the low pressure in the epidural space. Often in clinical practice such identification is not easy. The epidural space is variable in width. Before the surgeon could appreciate the loss of resistance, the needle tip may have crossed the epidural compartment and punctured the dura mater. Dural puncture is undesirable since it leads to complications such as xe2x80x9cpost dural puncture headachexe2x80x9d from loss of cerebrospinal fluid contained within the dural sac. Sometimes the epidural pressure may not be low, from various causes, and as a result, the surgeon may not appreciate the xe2x80x9closs of resistance to injectionxe2x80x9d being sought. The hollow of the needle may at times get clogged with tissue or a blood clot, further making the perception of low pressure in the epidural space difficult. In addition, pockets of loose areolar tissue or fat, outside the epidural space, may give a false sense of xe2x80x9closs of resistance to injectionxe2x80x9d. This may occur when the needle tip travels from firm tissue, such as a muscle or ligament, into a pocket of loose areolar or fat tissue. This leads to inadvertent injections outside the epidural space.
What would be desirable are improved needles for delivering material into the epidural space which allow for reproducibly locating the epidural space. What would be advantageous are methods for consistently delivering fluids to the epidural space not dependent upon the detection of low fluid pressure within the epidural space.
The present invention includes apparatus and methods for reproducibly and consistently locating the epidural space for delivery of fluids into the epidural space. The present invention utilizes an important anatomical component, the ligamentum flavum, to locate the epidural space. The ligamentum flavum forms the posterior wall of the spinal epidural space. It is a tough elastic ligament that runs longitudinally, connecting the lamina of adjacent vertebrae. Knowing that this ligament has been punctured informs the surgeon that the needle tip is located within the epidural space. Normally, since the ligamentum flavum is located several centimeters deep to the skin, the intervening ligamentous and muscle tissue render the puncturing of the ligamentum flavum hardly perceptible when using previous needles.
The present invention provides a flare or bulge near the distal tip of the epidural needle, which renders the ligamentum flavum distinctly perceptible to a surgeon as the needle is advanced through the ligament. Immediately after the needle point enters the ligament, the flare or bulge parts the elastic fibers of the ligament widely apart, giving a distinct feel of elastic resistance to the surgeon""s fingers advancing the epidural needle. Soon after the flare of the needle tip has passed through the ligament, the elastic fibers of the ligament collapse back around the shaft of a needle. This imparts a sense of xe2x80x9cgivexe2x80x9d or elastic recoil to the surgeon""s fingers holding the needle. The creation of a sense of elastic resistance followed by elastic xe2x80x9cgivexe2x80x9d or recoil, as the surgeon advances the epidural needle through the ligamentum flavum, is one aspect of the present invention. The present invention allows the epidural space to be localized irrespective of the pressure in the epidural space. The perception of puncturing the ligamentum flavum signals to the surgeon that the needle has entered the epidural space.
The present invention includes epidural needles having a distal bulge beginning proximally less than about 6 mm., preferable less than about 4 mm. from the distal tip. The present invention also includes methods for advancing an epidural needle, with one method including: providing an epidural needle having a distal tip and a bulge beginning proximally no more than about 4 millimeters from the distal tip; advancing the distal tip distally through the ligamentous structures while sensing a first resistance to advancement; continuing advancing the distal tip as the bulge penetrates the ligamentum flavum while sensing a second resistance to advancement greater than the first resistance to advancement; and stopping further advancement after a decrease in resistance is sensed relative to the first resistance to advancement.